1. Technical Field
This invention relates to polyoxymethylene compositions having increased elongation at break due to the inclusion therein of low levels of linear low density polyethylene (hereinafter referred to as "LLDPE").
Polyoxymethylene, also commonly referred to as polyacetal, compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent.
Compositions based on polyoxymethylenes of relatively high molecular weight, i.e., 10,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping and thermoforming. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance. However, in certain applications, it would be desirable to increase the elongation at break of the polyoxymethylene composition. By the present invention, the elongation at break of polyoxymethylene is found to be increased by the inclusion therein of low levels of a particular type of polyethylene; namely, LLDPE.
The compositions of the present invention are useful in those applications where it is desired to use a polyoxymethylene composition having increased elongation at break.
2. Background Art
The references below provide background information on polyolefin/thermoplastic resin compositions. While some of these references disclose of use of polyethylene in polyoxymethylene, none of the references teach that use of low levels of linear low density polyethylene, as opposed to other types of polyethylenes, in polyoxymethylene acts to increase elongation in polyoxymethylene. In fact, some of the references teach that the opposite effect is achieved, namely, that the inclusion of polyethylene into polyoxymethylene results in a decrease in the elongation of the polyoxymethylene.
U.S. Pat. Nos. 3,704,275; 3,795,715; and 3,980,734 disclose polyoxymethylene compositions containing homo- and co-polymers of polyethylene. No specific mention is made of linear low density polyethylene.
U.S. Pat. No. 3,969,313 discloses compositions comprised of a thermoplastic material, including polyoxymethylene, and 5-70 weight percent of a polyolefin composition comprised of a polyolefin and a reactive inorganic filler. The polyolefins which may be used include high density polyethylene, low density polyethylene, crystalline polypropylene, crystalline polybutene, poly-3-methyl-butene-1, poly-4-pentene-1, and copolymers comprising more than about 80 weight percent ethylene or propylene and less than 20 percent by weight of a comonomer selected from ethylene, propylene, butene-1, pentene-1, hexene-1, 3-methyl butene-1, and 4-methyl pentene-1.
U.S. Pat. No. 4,670,508 discloses that the inclusion of between 10-30 weight percent of ultra-high molecular weight polyethylene, or a maleic anhydride modified version thereof, into polyoxymethylene results in a decrease in the elongation at break of the polyoxymethylene from 24% to less than 10%.
Japanese Kokai Patent No. 49-40346 discloses that the elongation of polyoxymethylene is decreased by the inclusion therein of 10% of high density polyethylene (Table 1, Comparative Example 1), that the inclusion of 10% of linear low density polyethylene decreases the elongation of polyoxymethylene (Table V, Comparative Example 3), that the inclusion of 10% of low density polyethylene increases the elongation of polyoxymethylene slightly from 7% to 10% (Table V, Comparative Example 2), that the inclusion of 25% of high density polyethylene decreases that elongation of polyoxymethylene (Table VI, Comparative Examples), and that the inclusion of 20% high molecular weight polyethylene increases the elongation of polyoxymethylene from 7% to 12%.
Japanese Tokukai No. 62-253650 discloses compositions containing polyoxymethylene and 2-30% polyethylene, said compositions having improved squeak resistance. The reference teaches away from use of low levels of polyethylene and from use of polyethylenes having a melt index of less than 10. There is no mention of linear low density polyethylene in this reference, nor that the incorporation of linear low density polyethylene into polyoxymethylene improves the elongation at break of the polyoxymethylene.
U.S. Pat. No. 3,377,313 discloses polyoxymethylene compositions containing a dispersion of carbon black in a thermoplastic polymeric material, such as, amongst others, a polyolefin, including polyethylene. No specific mention is made of linear low density polyethylene.